In the conventional fourdrinier papermaking process, a water slurry, or suspension, of cellulosic fibers (known as the paper "stock") is fed onto the top of the upper run of an endless belt of woven wire and/or synthetic material that travels between two or more rollers. The belt, often referred to as a "forming fabric", provides a papermaking surface on the upper surface of its upper run which operates as a filter to separate the cellulosic fibers of the paper stock from the aqueous medium, thereby forming a wet paper web. The aqueous medium drains through mesh openings of the forming fabric, known as drainage holes, by gravity or vacuum located on the lower surface (i.e., the "machine side") of the fabric.
After leaving the forming section, the paper web is transferred to a press section of the paper machine, where it is passed through the nips of one or more pairs of pressure rollers covered with another fabric, typically referred to as a "press felt." Pressure from the rollers removes additional moisture from the web; the moisture removal is often enhanced by the presence of a "batt" layer of the press felt. The paper is then transferred to a drier section for further moisture removal. After drying, the paper is ready for secondary processing and packaging.
Typically, papermakers' fabrics are manufactured as endless belts by one of two basic weaving techniques. In the first of these techniques, fabrics are flat woven by a flat weaving process, with their ends being joined to form an endless belt by any one of a number of well-known joining methods, such as dismantling and reweaving the ends together (commonly known as splicing), or sewing on a pin-seamable flap on each end or a special foldback, then reweaving these into pin-seamable loops. In a flat woven papermakers' fabric, the warp yarns extend in the machine direction and the filling yarns extend in the cross machine direction. In the second technique, fabrics are woven directly in the form of a continuous belt with an endless weaving process. In the endless weaving process, the warp yarns extend in the cross machine direction and the filling yarns extend in the machine direction. As used herein, the terms "machine direction" (MD) and "cross machine direction" (CMD) refer, respectively, to a direction aligned with the direction of travel of the papermakers' fabric on the papermaking machine, and a direction parallel to the fabric surface and transverse to the direction of travel. Both weaving methods described hereinabove are well known in the art, and the term "endless belt" as used herein refers to belts made by either method.
Effective sheet and fiber support and an absence of wire marking are important considerations in papermaking, especially for the forming section of the papermaking machine, where the wet web is initially formed. Wire marking is particularly problematic in the formation of fine paper grades, as it affects a host of paper properties, such as sheet mark, porosity, see through, and pin holing. Wire marking is the result of individual cellulosic fibers being oriented within the paper web such that their ends reside within gaps between the individual threads or yarns of the forming fabric. This problem is generally addressed by providing a permeable fabric structure with a co-planar surface which allows paper fibers to bridge adjacent yarns of the fabric rather than penetrate the gaps between yarns. As used herein, "co-planar" means that the upper extremities of the yarns defining the paper forming surface are at substantially the same elevation, such that at that level there is presented a substantially "planar" surface. Accordingly, fine paper grades intended for use in carbonizing, cigarettes, electrical condensers, quality printing, and like grades of fine paper, have typically heretofore been formed on very finely woven or fine wire mesh forming fabrics.
Such finely woven forming fabrics, however, often are delicate and lack dimensional stability in either or both of the machine and cross machine directions (particularly during operation), leading to a short service life for the fabric. In addition, a fine weave may adversely effect drainage properties of the fabric, thus rendering it less suitable as a forming fabric.
To combat these problems associated with fine weaves, multi-layer forming fabrics have been developed with fine-mesh yarns on the paper forming surface to facilitate paper formation and larger yarns on the machine contact side to provide strength and longevity. As examples, U.S. Pat. No. 4,709,732 discloses a dual layer forming fabric for use in a papermaking process, U.S. Pat. No. 5,025,839 discloses a two-ply forming fabric with zig-zagging MD yarns, and U.S. Pat. No. 4,605,595 teaches a two ply forming fabric with a two-shaft, twill or satin weave pattern.
Although double-layer fabrics have proven to be effective forming fabrics for many applications, they can be expensive to manufacture. Also, different paper varieties are generally produced on different types of fabrics. For example, a high grade paper, such as that used in magazines and printers, is typically produced on a considerably different fabric than a tissue paper, which has significantly more lenient surface standards. Accordingly, fabric designers are constantly searching for new designs that provide an appropriate balance of performance characteristics and cost.
One example of a double-layer fabric which is suitable for forming tissue paper is disclosed in U.S. Pat. No. 5,025,839 to Wright. This fabric employs MD yarns that are interwoven with the machine side CMD yarns in an "under 1/over 1/under 1/over 5" pattern, the result of which is the MD yarns producing a "zigzag" effect that reportedly improves drainage. The MD yarns are interwoven with the CMD yarns of the paper side of the fabric in an "over 1/under 2/over 1/ under 12" repeating pattern, with the MD yarns interlacing with the machine side CMD yarns in the "under 12" sections.
Unfortunately, this fabric has proven to be prone to "twinning" of its paper side CMD yarns in the "under 2" positions of the pattern (the positions between the locations where the MD yarns pass over the paper side CMD yarns to form paper side "knuckles"). Twinning is an effect in which adjacent paper side CMD yarns tend to reside near one another rather than being spaced apart a uniform distance. This is caused by tension in the machine direction yarns due to the "under 1/over 1/under 1" portion of the machine side pattern, which is a tension-inducing configuration. This tension forces the "under 2" paper side CMD yarns together in a "twinned" configuration. Twinning can result in uneven drainage through the paper side layer due to the disparity in drainage hole size.